Sensitivity of the European LGM climate to North Atlantic sea-surface temperature

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Sensitivity of the European LGM climate to North

Atlantic sea-surface temperature

Sophie Pinot, Gilles Ramstein, Isabelle Marsiat, Anne de Vernal, Odile

Peyron, Jean-Claude Duplessy, Maria Weinelt

To cite this version:

Sophie Pinot, Gilles Ramstein, Isabelle Marsiat, Anne de Vernal, Odile Peyron, et al.. Sensitivity of

the European LGM climate to North Atlantic sea-surface temperature. Geophysical Research Letters,

American Geophysical Union, 1999, 26 (13), pp.1893-1896. �10.1029/1999GL900361�. �hal-02957898�

GEOPHYSICAL RESEARCH LETTERS, VOL. 26, NO. 13, PAGES 1893-1896, JULY 1, 1999

Sensitivity of the European LGM climate to North

Atlantic sea-surface temperature

Sophie

Pinot,

x Gilles

Ramstein,

• Isabelle

Marsiat,

2 Anne de Vernal,

a

Odile Peyron,

• Jean-Claude

Duplessy,

x Maria Weinel•

s

Abstract. Recent reconstructions of Sea-Surface Tem-

peratures

(SSTs)

for the Last Glacial

Maximum

(LGM,

21kyr BP) based on foraminifera

and dinoflagel!ate

ter understand the impact of such a warm north At-

lantic on the global LGM climate, we used two differ-

ent AGCMs to perform sensitivity studies. With the

new, warmer SSTs, both models simulate a hydrolog-

ical cycle and temperatures very different from those

obtained with the CLIMAP boundary conditions. The

most noticeable differences occur in winter over North

America and Siberia whereas southern Europe is only

weakly affected at all seasons. Whichever the condi-

tions prescribed over the north Atlantic, both models underestimate the large cooling recorded by continen-

tal proxy data over the Mediterranean Basin.

Introduction

Many AGCM simulations (e.g. [Broccoli and Man-

abe, 1987]) have shown

the impact on the general

at-

conditions and sea-ice cover prescribed over the north Atlantic and the presence of Fennoscandian and Lauren-

tide large ice-sheets, perturbed the extratropical atmo-

spheric

circulation

(e.g. [COHMAP members,

1988]).

el., 1992],

coccoliths

[Hebbeln

et al., 1994],

dinoflagel-

lates [de Vernal

et al., 1997]

and biomarkers

[Rosell-

1Laboratoire des Sciences du Climat et de l'Environnement,

UMR CEA-CNRS, Gif-sur-Yvette, France.

2Dpt. of Meteorology, University of Reading, UK.

aGEOTOP, Universit6 du Qu6bec & Montr6al, Canada.

4CEREGE, CNRS, Aix-en-Provence, France. 5Geologish-palaontologishes Institut, Kiel, Germany.

Copyright 1999 by the American Geophysical Union.

Paper number 1999GL900361.

0094-8276/99/1999GL900361 $05.00

CLIMAP [1981, hereafter

CLIMAP]. These new recon-

warmer than the CLIMAP reconstructions, with less

sea-ice extent.

We have performed several experiments to test the sen- sitivity of two different AGCMs to changes in LGM north Atlantic SSTs, to infer the impact of a warmer north Atlantic on the European terrestrial environment.

Simulation results for PD and LGM

using CLIMAP

SSTs

PD experiments

The AGCMs used are the LMD5 and the UGAMP models. The differences between both models are dis-

cussed in detail in the PMIP documentation (www- pcmdi.llnl.gov/pmip). Both models are able to simu- late the extratropical Present-Day (PD) climate which

is strongly influenced by the thermal contrast between ocean and land, and by orography. Over Europe, the main features of the precipitation and temperature fields are in reasonable agreement with available climatologies

[Masson et al., 1999].

LGM experiments using CLIMAP SST

The two simulations described in this study use the same set of boundary conditions for the LGM simu-

lation, as recommended by PMIP (Paleoclimate Mod-

eling Intercomparison

Project) [Pinot et al., in press]:

from Peltier [1994]

and SSTs

from CLIMAP [1981]

(Fig

and the LGM runs except over the ice-sheets.

Both the LMD5 [e.g. Fabre et al., 1998] and the

UGAMP [Dong and Valdes,

1998] models

reproduce

the major characteristics of the LGM climate found in

previous simulations (an important cooling over the ice- sheets and over the north Atlantic due to the ice cover,

a smaller temperature decrease in the equatorial area

[Pinot et al., in press] and a reduced globally hydrolog- ical cycle).

On Europe, the common response of both models to

full glacial boundary conditions, is mainly explained by

the presence of extensive sea-ice over the Atlantic ocean.

This forcing factor induces adiabatic cooling over the

northern Atlantic and determines a zonal circulation

1894 PINOT ET AL.- LGM CLIMATE SENSITIVITY TO NORTH ATLANTIC SST

I

III

I I

.

J

in winter, because the sea-ice cover reduction in win- ter increases the interaction between atmosphere and

ocean.

It should be pointed out that the composite SST re- construction is based on different micropaleontological

indicators (the dinoflagellate cysts and the planktonic foraminifers). They still need an intercalibration before

a consistent picture of the sea-surface conditions over the north Atlantic during the LGM can be obtained. This evaluation is the object of the on-going EPILOG project, and is beyond the scope of this paper. The ma- jor aim of this new SST reconstruction is to investigate

the impact of a warm LGM north Atlantic on the Eu- ropean climate in contrast with the cold north Atlantic

situation depicted by CLIMAP.

Figure 1. top' LGM CLIMAP [1981] SST (Fig 2a and 2b); middle: new warm SST LGM reconstruction (Fig 2c and 2d); bottom: differences between the two reconstructions (Fig 2e and 2f). The results are given

on the LMD5 grid, on the right for the summer season

and on the left for the winter season (K).

over north Atlantic and western Europe

al., 1999].

[Kageyaraa et

A sensitivity experiment for the LGM

The composite SST data set

We have first built a new sea-ice data set, merging the

reconstructions

based

on dinofiagellates

cysts

[de Vernal

foraminifera

faunal changes

[Weinelt et al., 1996]

for

SST data set consistent with the first one and using the SST reconstructions from both indicators. Along the 20øW meridian, differences between the warmer SSTs

based on dinofiagellate and the colder ones based on

foraminifera have been smoothed. Everywhere else, the CLIMAP SSTs have been used, with.a gradual transi-

tion to the other data sets at 10øN (the differences

at

The differences between CLIMAP and the new LGM

SST reconstructions

are larger

in summer

(Fig lf) than

reduced over the northeastern Atlantic ocean in summer and over the central and northeastern Atlantic ocean in

winter. The SSTs are warmer than CLIMAP by up to

2øC in winter and 8øC in summer. Despite the SST

changes being more important in summer, the stronger impact of this new SST data set on the atmospheric

circulation and on the continental temperatures occurs

Results of sensitivity experiments

The same LGM sensitivity experiment has been per- formed with the LMD5 and the UGAMP models, mod-

ifying only the north Atlantic SSTs. In this section, the simulation using the CLIMAP SSTs serve as our

reference LGM simulation.

Hydrological cycle. Over the north Atlantic, the

warmer SSTs (less sea-ice) induce a strong change in the rate of evaporation at the surface. However, most

of the increase in the transfer processes constituting the hydrological cycle remains local and only contributes to reinforce local recycling For the CLIMAP LGM sim-

ulations, local recycling is equal to 88% for LMD5 and

77% for UGAMP. In contrast, it reaches values of 98% for LMD5 and 86% for UGAMP for the warmer SST ex- periments. The evaporation increase when using warm

LMCELMD5 model

I I ,,, i t II I I

UGAMP model

!

18o 1sow 12ow t,ow 6ow .--low

-10 -4 -2 -0.5

t • ,.J t i

1 3 5 15

-15 -5 -3 -1 0.5 2 4 !0

Figure 2. Temperature of the coldest month (K) in

the northern hemisphere, differences between the ex-

periment with CLIMAP SST and the experiment with the new warm SST reconstruction.

PINOT ET AL.: LGM CLIMATE SENSITIVITY TO NORTH ATLANTIC SST 1895

Table 1. Energy budget over northern Atlantic ocean

(60øW 5øE - 50øN 90øN) and over Europe (10øW 50øE - 30øN 60øN) for the PD, LGM with CLIMAP SST (LC) and LGM with the new warm SST reconstruc- tions (LW). Fluxes are counted positively for the atmo-

sphere, LHF - Latent heat flux, SHF - sensible heat flux, HBA = heat budget for the atmosphere, GT - ground temperature. LMD5 UGAMP ATL. OC. PD LC LW PD LC LW LHF 34 8 31 41 12 35 SHF 16 2 22 26 9 30 HBA -40 -75 -37 -44 -72 -33 GT 5.8 -13.2 -3.2 3.7 -15.4 -5.3 EUROPE PD LC LW PD LC LW LHF 41 27 29 24 13 15 SHF 37 35 34 57 49 49 HBA -20 -24 -24 -23 -36 -36 GT 13.9 6.5 7.2 13.2 4.2 6.2

and UGAMP models depict a strong cooling over NPA

(10øC for LMD5 and 12øC for UGAMP) whereas SPA

only experiences a cooling of around 8øC and 10øC re- spectively. These models are therefore far from repro- ducing the large temperature decrease estimated from

pollen reconstructions.

On the other hand, using warmer north Atlantic SSTs, LMD5 and UGAMP models simulate winter tempera- tures 5øC warmer than those estimated using the CLIMAP boundary conditions over NPA and 1 or 2øC higher

MTCO over SPA (Fig 2) .

The important result is that the terrestrial tempera- ture over SPA and NPA simulated by both models, un-

der both cold (CLIMAP) or warm (composite new data set) north Atlantic SST conditions, are largely under-

estimated. As far as we do not expect a north Atlantic SST reconstruction drastically outside these limits, we strongly believe that the discrepancy between model re- suits and terrestrial data for southern Europe at LGM

is not due to the uncertainty that still remains on the north Atlantic SSTs reconstruction.

SSTs at LGM is associated with more latent heat re- lease, which reaches values close to those obtained in

the PD simulation

(22W/m

2 for LMD5, 30W/m

2 for

UGAMP). Conversely, the latent heat release is strongly

damped

when

using

CLIMAP SSTs

(SW/m 2 for LMD5,

12W/m 2 for UGAMP) (Table 1). Using

warm compos-

heat flux reaching

22W/m

2 for LMD5 and 30W/m

2 for

UGAMP. These values are even higher in LGM simula-

tion using the new SST data set than for PD (Table 1). Over Europe, the energy budget of the atmosphere (Ta- ble 1) is similar to the reference LGM CLIMAP SST

run. As shown before, all the changes in the hydrologi- cal cycle over the Atlantic ocean essentially lead to more local recycling. Indeed, the latent heat transport from the Atlantic ocean toward Europe is reduced for both LGM simulations when compared to the PD simulation. Temperature changes. We observe that the tem- peratures, when warm Nordic SSTs are prescribed, are

colder in winter in Siberia and North America whereas the temperature over Europe are warmer (these changes are associated to changes in circulation). Nevertheless, over •the Mediterranean basin both models are weakly affected (Fig 2) by the changes in boundary conditions.

Comparison of LGM simulations with pollen data over Southern Europe. New estimates of cli- matic conditions prevailing over the north of the Py-

renees-Alps line (NPA, >40øN) and south of that line

(SPA, including

sites

in Italy, Greece

and Turkey) have

Using CLIMAP SSTs as boundary conditions, the LMD5

Discussion and conclusion

We have tested the sensitivity of both the LMD5 and UGAMP AGCMs to warmer than CLIMAP [1981]

LGM north Atlantic SSTs using new reconstructions based on foraminifera and dinofiagellates. We used

two different AGCMs in order not to obtain model-

dependent results. We found that these new SSTs have a major impact on the hydrological cycle and energy budget over the north Atlantic ocean, but a rather small impact over southern Europe: these re- sults show the weak sensitivity of the Mediterranean basin terrestrial temperatures simulated by both mod- els to the prescribed LGM north Atlantic SST changes.

Whichever north Atlantic SST reconstruction is used (cold: CLIMAP or warm: new SST data set), model

results underestimate the cooling inferred from pollen

data.

Among the causes for such a discrepancy, a plausible explanation is the vegetation impact. This forcing fac- tor is not accounted for in these experiments mainly because no realistic global reconstruction is currently

available. Previous sensitivity studies (using an asyn- chronously coupled global atmosphere-biome model [Ku- batzki and Claussen, 1998] or using an AGCM with

an ice-age vegetation reconstruction as boundary con-

ditions [Crowley and Baum, 1997]) showed that the

climatic impact of vegetation changes may explain a cooling over the Mediterranean basin. Additional ex- periments also suggest that colder than CLIMAP SST in the north Pacific and the tropics, as well as ice-sheets

higher than those estimated by Peltier [1994] may also contribute to explain differences between models and

data.

Acknowledgments. This work have been supported by CEA, CNRS and EC under contracts number ENV4-

1896 PINOT ET AL.: LGM CLIMATE SENSITIVITY TO NORTH ATLANTIC SST

CT95-0075. We thank the EPD (European Pollen Database)

for providing the pollen reconstructions. The sea-surface temperature data set from the German group of Kiel was kindly provided by Dan Seidov. The authors thank Ger- hard Krinner and Masa Kageyama for fruitful comments on

the manuscript.

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J.-C. Duplessy, S. Pinot and G. Ramstein, Laboratoire

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O. Peyron, CEREGE, CNRS, B.P. 80, 13545 Aix-en-

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M. Weinelt, Geologish - palaontologishe Institut,

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(Received December 7, 1998; revised April 2, 1999; accepted April 30, 1999.)